| In recent years,with the rapid development in petroleum refining,ethylene production and MTO industries,the production of by-product C4 resources has been increasing considerably.Selective hydrogenation of C4 hydrocarbons is a key technique that bridges upstream and downstream in the utilization of C4 hydrocarbons,and its core is designing an efficient and stable hydrogenation catalyst.In this dissertation,Pd/Al2O3 and Ni/Al2O3 catalysts were developed for selective hydrogenation of butadiene,1-butene and higher alkyne,respectively.Besides,a second component,such as Ag,S and B,was introduced to improve catalyst selectivity.DFT calculations and structural characterizations were performed to identify the dominant factors that determine the catalyst performance in catalyzing selective hydrogenation of C4 hydrocarbons,based on which the catalyst structure was tailored toward the improved performance.Furthermore,the process conditions were optimized and the catalyst stability tests were carried out.The main results are described as follows:(1)Pd/Al2O3 catalysts were prepared by excessive impregnation method for the selective hydrogenation of a C4 mixture(rich in 1-butene)containing trace butadiene.The effects of support properties and catalyst preparation conditions on the structure and performance of the catalyst were investigated.The results showed that compared to the strong acid support,Pd/Al2O3 catalyst using a support with a considerable amount of medium acid sites have a smaller particle size of 4.0 nm,narrower distribution and higher electron density.This catalyst provides a higher active surface area and weakens the adsorption of olefins on it,so the catalyst shows better hydrogenation activity of butadiene and 1-butene selectivity.The catalytic performance of Pd/Al2O3 also depends on its Pd geometric distribution and dispersion.For the best catalyst,Pd is evenly distributed in the shell layer of 10-60 ?m on the catalyst,and the Pd dispersion is 16%?18%.When the thickness of the Pd layer is narrower,the density of Pd cluster is higher,which is not conducive to the catalyst selectivity;when it is wider,the diffusion path length of reactants is increased and then the hydrogenation activity is reduced.If the Pd dispersion is too high,it means that the number of low coordination active sites on the surface increases and then the adsorption of reactants is too strong.This not only inhibits the hydrogenation of butadiene,but also reduces the selectivity of 1-butene.Under the optimized conditions,the optimal Pd/Al2O3 catalyst demonstrates a 1000-h stability,the residual butadiene is less than 10 ppm,and the yield of 1-butene is more than 95%.The selective hydrogenation of butadiene in the presence of butene-1 was investigated over Ag promoted Pd-based catalyst.The results showed that Ag can significantly enhance activity and selectivity of the Pd-based catalysts in the selective hydrogenation of C4 fraction,achieving a butadiene residue lower than 10 ppm and 1-butene selectivity of 98.6%.It was found that the introduction of Ag suppresses the formation of subsurface hydrogen and decreases the Pd dispersion and electron density,resulting in the improved catalyst with an increase both in the conversion of butadiene and the yield of 1-butene.Moreover,the Pd-Ag/Al2O3 catalysts show the composition-dependent performance for C4 hydrogenation,as the extent of structure modification of Pd sites is dependent on the fraction of Ag.(2)Using nickel-based catalyst was proved to be a promising way of converting 1-butene to 2-buene.Compared to nickel nitrate as Ni precursor,Ni/Al2O3 catalyst derived from basic nickel carbonate shows dispersed Ni particles with smaller size and higher Ni electron density,having better hydroisomerization performance of 1-butene.Furthermore,enhanced selectivity was achieved over Ni/Al2O3 catalyst by adding sulfur promoter.Characterizations reveal that the part of Ni active sites combined with S2-results in more efficient catalyst activation procedure and reducing the probability for over-hydrogenation,meanwhile,increasing the tendency for isomerization of 1-butene.Under the optimum conditions,the superior performance with a butadiene conversion of 100%,2-butene selectivity of 97.2%and 1-butene conversion of 92.6%was obtained over Ni-S/Al2O3 of S/Ni(w/w)=0.1.The catalyst performance could be further improved by increasing the pore size.(3)The catalytic hydrogenation technology has a promising application in converting waste emissions(acetylenic contaminants)into valuable chemicals,thus,achieving the goals of energy saving and emission reduction.Ni-Ag/Al2O3 catalyst was found to have a higher acetylenes hydrogenation activity over Ni-based catalyst.Physicochemical characterizations and density functional theory calculations indicate that the addition of Ag does not change the particle size of Ni nanoparticles significantly,but isolates the Ni ensemble sites and increases the Ni electron density.The latter two factors weaken the adsorption strength of reactants and reaction intermediates.The weaker adsorption of alkynes decreases the coverage of alkynes and then provides more free active sites for the adsorption of hydrogen,while inhibits the formation of heavy depositions.Therefore,the Ni-Ag/Al2O3 catalyst demonstrates a high durability during 700-h catalytic reaction test.(4)To explore structural effects of Ni-based catalysts in catalyzing partial hydrogenation of alkynes,the Ni-B/Al2O3 catalyst was prepared by KBH4 reduction at room temperature and long-chain alkyne hydrogenation was taken as the reaction system.The results indicate that Ni-B/Al2O3 shows a 1-octene selectivity of 91%at the 1-octyne conversion of 99%.Physicochemical characterizations reveal that the introduction of chemical reduction method results in the formation of small Ni nanoparticles,the limited amount of Ni active sites for hydrogenation and the presence of electron-enriched Ni species.Owing to the decreased Ni particle size,Ni-B/Al2O3 catalyst exhibits a higher activity compared to the Ni/Al2O3 catalyst with more active sites but larger size.The electron-enriched Ni species weaken the adsorption of 1-octene and thus suppress the hydrogenation of 1-octene,which contributes to the obviously enhanced selectivity. |
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